There is a growing interest in digital radiography which offers faster results, easier processing, and easier storage than traditional x-ray photographic techniques. Applications for this form of radiography include medical diagnostics, port security and non-destructive testing.
Radiation detection by digital radiography includes both indirect and direct detection. Indirect detection uses a thin photodiode, usually an amorphous silicon p-i-n diode, to detect light from a phosphor conversion screen which is placed in direct contact with an array detector. The emitted light is produced by the interaction of the detector and x-ray or gamma ray photons.
Direct detection uses a thick x-ray sensitive photoconductor to absorb the x-rays and create the signal charge. Generally, direct detection is considered the better approach because image blur is expected to be small. Furthermore, direct charge collection offers the better possibility for high efficiency x-ray or gamma ray conversion. The photoconductor of a direct detection apparatus must combine high ionization rate, high charge collection, low dark current, high modulation transfer function (MTF) and the ability to be deposited over a large area. This last feature requires the use of either an amorphous or polycrystalline material.
Imaging detectors first employed single crystals, but while single crystals can give excellent single photon detection, their cost and the difficulty in obtaining crystals of the size required for large area detectors have reduced their usefulness.
Recently, polycrystalline films of photoconductive wide band gap semiconductor materials have been used instead of single crystals for large area detectors. The principal wide band gap semiconductor material used has been Se, although other materials such as Pbl2 and Hgl2 have been tried. Generally, single crystals have better detection properties and are preferable to polycrystalline materials for detection. For example, single crystal Hgl2 detectors perform better then polycrystalline Hgl2 detectors because, among other things, the mobility-lifetime (mu-tau) product of single crystals is of the order of 10−4 cm2/Vsec compared to that of films where the product is 10−5 cm2/Vsec or less. A higher mu-tau allows for better charge collection.
Notwithstanding the above, large area imaging systems typically are fabricated as polycrystalline array detectors. Semi-conducting polycrystalline material is generally deposited by any of a variety of methods, but typically by physical vapor deposition (PVD) or plasma enhanced chemical vapor deposition (PECVD). A description of the deposition of polycrystalline materials on high energy detectors by PVD is described inter alia in M Schieber et al., Medical Imaging Pro. SPIE, Denver 1999, Vol. 3770 (1999), 146–155, which is herein incorporated by reference in its entirety.
FIG. 1, to which reference is now made, shows a block diagram of a typical prior art imaging system using either single crystal or polycrystalline pixelated array detectors. The electronics used with these detectors are configured to produce images by using the photogain and dark current readings of the system.
FIG. 1 shows a prior art schematic block diagram of a pixelated array and electronics appended thereto which can be used to detect and image high energy radiation. The diagram indicates a pixelated array 12 to which readout amplifiers 14 and gate drivers 16 are connected. High energy photons impinge on photoconductive material deposited on the pixels of array 12; the photons generate electron/hole pairs, which are collected by a charge collecting substrate, often a thin film transistor (TFT). Depending on the selected bias either the electrons or holes can be collected.
The electronics take the image acquired at every addressed pixel of the array and organize the sequential transfer of digital data to a processor 22, usually a computer processor. The gate addressing lines of the array are connected to gate drivers 16 that provide sequential addressing pulses. These integrated circuits (ICs) usually have 100–200 channels so that several are required for a large area, high-resolution imager. The readout amplifiers 14 sense the charge on the data lines and then amplify the analog signal providing it to analog-to-digital (A/D) converters 18. The signal is than digitized and passed to a computer image processor 22 for processing and display. The image processor 22 processes the data and sends it for display on image display 24. Additionally, there is a control logic unit 20A that provides timing for the operations and a power regulation unit 20B which provides filtered dc power to the system.
Reference is now made to FIG. 2 which illustrates a typical prior art pixel equivalent circuit 40 for a photoconductor detector plate 44 in an x-ray imaging system. Circuit 40 shows one detector electrode 58 connected to a bias voltage 50. The second electrode 56 is connected to a storage capacitor 46 and a switching thin-film transistor (TFT) 42. Storage capacitor 46 is connected to a ground plane 48. Thin-film transistor 42 is connected to both a gate line 62 and a data line 60, with data being fed to a readout unit (not shown) through data line 60.
Reference is now made to FIG. 3 where another schematic representation of the prior art electronics and photodetector used for single photon detection as discussed herein is shown. FIG. 3 can be considered as a conflation of FIGS. 1 and 2 shown above. Similarly, items in the Figure have been given numerals identical with those in FIGS. 1 and 2 and therefore the description of these items and their function will not be repeated. Area 65, circumscribed by a broken line, represents that portion of the system found in each pixel of array 12 (FIG. 1). The TFT of each pixel is connected to a gate addressing line 62 and a data addressing line 60 which are connected to gate drivers 16 and readout amplifiers 14 of FIG. 1.
X-ray detectors fabricated from single crystal photoconductive semiconductors, with typical dimensions of 1×1×1 mm, are known to give good energy resolution for x-ray detection. To date, the only detection apparatus for detecting single x-ray photons in, for example, the 80–90 keV range, has been fabricated from single crystal photoconductive semiconductors and has dimensions typically about 1×1×1 mm. Attempts have been made to make single crystal array detectors which typically have 64×64 pixels, each pixel being about 100–125 microns×100–125 microns. However, this number and size of pixels is too small for radiography and is limited by the problems of making large single crystals and the methods by which the crystals are bonded to silicon substrates. Such arrays are discussed in Nuclear Instruments and Methods a, vol. 38, 1996 pp 252–255 and 262–265.
Flat panel detectors constructed essentially as described in FIGS. 1–3 above are becoming widely used for x-ray and medical imaging. These often comprise several million pixels, each pixel the size of from about 100 to about 300 μm, and the overall array size can reach up to 40×40 cm. Such detectors generally are made using polycrystalline or amorphous photoconductive materials. Single photon x-ray detection has not been achieved with these systems because the signal-to-noise ratio has been too low. This is due to a combination of low x-ray conversion efficiency in non-single crystal detectors and the high noise electronics used in these systems.
Present day array detectors, whether employing polycrystalline or amorphous photoconductive materials, all operate in a charge collection mode where they collect charge over time with one readout per image produced. Detectors operating in such a mode generally require higher fluxes and longer exposures than would be required by a successful single photon detector.
There are uses in medicine and elsewhere for detectors which must detect low flux high-energy radiation. A single photon large area detector array having smaller pixel size, and hence higher resolution with better sensitivity, would fill a need and would be applauded by the medical community.
An example of the state of prior art high energy radiation direct detectors that make use of polycrystalline and single crystal photoconductive materials is discussed in the following publications and references cited therein:
Comparison of Pbl2 and Hgl2 for direct detection active matrix x-ray image sensors, R A Street et al, J. Appl. Phys., 91,3345 (2002);
Approaching the Theoretical X-ray sensitivity with Hgl2 Direct Detection Image Sensors, R. A. Street, et. al, SPIE Conf. Proc. 4682, p 414, 2002.